use std::cell::OnceCell;
fn main() {
    let cell = OnceCell::new();
    assert!(cell.get().is_none()); // true
    let value: &String = cell.get_or_init(|| "Hello, World!".to_string());
    assert_eq!(value, "Hello, World!");
    assert!(cell.get().is_some()); //true
    println!("{}", cell.get().is_none());//false
}
